Here's his response in the New York Times.
I also found out yesterday that this is not the first time Ec10 has been protested. The last time it happened, they apparently got more done from the protest than this time. Here's a letter to the editor by the kid that spear-headed the movement back then.
I've been meaning to write something more about Mankiw's textbook - and perhaps I will at some point. I think it's more Keynesian in its discussion of fiscal and monetary policy than a lot of people give it credit for - but in a way that these protesters probably wouldn't appreciate (the section relies on a liquidity preference theory of the interest rate alone. Of course he discusses loanable funds theory earlier, but I was a little surprised to say that. Of course this is taking Keynes's position againt both Hicks and the classicals.
He's still got that pesky diagram labeling a labor surplus as "unemployment".